Travel time methods make use of the physical law, whereby the traveled distance is equal to the product of travel time and propagation velocity. In the case of fill level measurement, the traveled distance corresponds to twice the distance between a reference point of the transmitter and the surface of the fill substance. The useful, or wanted, signal, thus the signal reflected on the surface of the fill substance, and its travel time are preferably ascertained on the basis of the so-called echo curve, which represents, in analog or digital form, amplitude of the echo signals as a function of distance from the transmitter. The fill level itself is then obtained from the difference between the known distance of the reference point of the transmitter to the floor of the container and the distance of the surface of the fill substance to the reference point of the transmitter as determined by the measurement.
All known methods can be applied, which enable relatively short distances to be ascertained by means of reflected measuring signals. If the measuring signals are microwaves, then both the pulse radar method and also the frequency modulation continuous wave radar method (FMCW-radar) can be used. Microwave measuring devices using freely radiated, pulsed measuring signals are sold by the assignee, for example under the mark, MICROPILOT. Microwave measuring devices, which guide measuring signals along a conductive element, into, and out of, a container, are sold by the assignee under the mark, LEVELFLEX.
A device type, which works with ultrasonic signals, is sold by the assignee, for example, under the mark, PROSONIC.
Measurement accuracies in the millimeter range are obtained with fill-level measuring devices ascertaining fill level of a medium in a container via the travel time of measuring signals. Apart from the providing of highly accurate measured values, travel-time, fill-level measuring devices are also used in increasing degree for obtaining measured values in areas where very high safety requirements must be fulfilled. By way of example, referenced here are measuring devices fulfilling requirements for functional safety according to IEC 61508 or measuring devices serving for overfill protection according to the German water management law (WHG).
In order, in the case of such critical applications, to assure that the measuring device functions, as such, truly reliably, repeated checking of the correct functioning of the measuring device is an absolute necessity. Especially, it must be regularly checked whether the measuring device is delivering a correct measured value, in the case of use as overfill protection, as characterized by a maximum allowable fill level, or in the case of use as protection against running empty, as characterized by a minimum allowable level.
According to long established practice, in the checking of the measuring device, a certain fill level of the fill substance in the container—in the mentioned cases, the maximum or minimum allowable fill level—is approached in regular increments. In such case, it is checked whether the measuring chain is functioning properly, thus, for example, whether the required alarm is being triggered. Fill-level measuring devices of today can be applied in containers of height up to 30 m. If one takes into consideration the size of the container to be monitored, then it is easy to see that this known and practiced method is very complicated and thus expensive. Especially disturbing is the fact that, for performing the known test method, the running process must be interrupted for the length of the review.